justone:
...get the 5/8" bore sprocket and get a 7/16" to 5/8" bronze sleeve bearing (6381K465) and with a hacksaw cut away for the key.
I would suggest cutting all the way through one side and then grinding a tall key that reaches between the motor shaft and keyslot on the sprocket.
If you try cutting a keyway without an arbor press and keyway broach I think you'll find it nearly impossible. You can't do it with a hacksaw and trying to file it is incredibly painstaking.